Quinolones inhibit DNA religation mediated by Staphylococcus aureus topoisomerase IV - Changes in drug mechanism across evolutionary boundaries

Quinolones are the most active oral antibacterials in clinical use and act by increasing DNA cleavage mediated by prokaryotic type II topoisomerases. Although topoisomerase TV appears to be the primary cytotoxic target for mo st quinolones in Gram-positive bacteria, interactions between the enzyme an d these drugs are poorly understood. Therefore, the effects of ciprofloxaci n on the DNA cleavage and religation reactions of Staphylococcus aureus top oisomerase TV were characterized. Ciprofloxacin doubled DNA scission at 150 nM drug and increased cleavage similar to 9-fold at 5 mu M. Furthermore, i t dramatically inhibited rates of DNA religation mediated by S. aureus topo isomerase TV. This inhibition of religation is in marked contrast to the ef fects of antineoplastic quinolones on eukaryotic topoisomerase II, and sugg ests that the mechanistic basis for quinolone action against type II topois omerases has not been maintained across evolutionary boundaries. The appare nt change in quinolone mechanism was not caused by an overt difference in t he drug interaction domain on topoisomerase TV. Therefore, we propose that the mechanistic basis for quinolone action is regulated by subtle changes i n drug orientation within the enzyme drug DNA ternary complex rather than g ross differences in the site of drug binding.